The invention relates to a method of determining the optical quality of and detecting faults in flat glass and other optically transparent materials.
A method is known for determining the optical quality of flat glass, especially of float glass, wherein a video camera is arranged to monitor an illuminating device either through the glass or by observing the reflection of the illuminating device on the glass. In this case, the focus of the video camera is on the glass and the sheet, respectively. In the process, the video camera generates signals in dependence on the quality of the glass. These signals will subsequently be evaluated.
FIG. 1 illustrates a method according to the state of the art. A video camera 1 or cell camera is provided to monitor, through a glass sheet 2, an illuminating device 3 having a dark field arranged thereon.
In case of a faultless material of the glass 2, the camera 1 will view the dark field 4. In case of a fault, the optical effect of the glass 2 will distort and/or deflect the field of view of the camera 1. If this effect is so large that the field of view of the camera 1 is caused to shift partially or wholly into the bright field 5, the video signal will undergo corresponding changes.
The dark field 4 must always be sufficiently large to prevent the field of view of the camera 1 from being shifted into the bright field 5 also due to vibration or bending (e.g. under the influence of temperature). For this purpose, the sensitivity of the system is limited by dead zones.
As long as the field of view deflected by a fault is located in the borderline region between the dark field 4 and the bright field 5, the amplitude of the error signal is dependent on the amount of the deflection. Since, however, the amplitude is also influenced by contamination of the sheet 2 under testing, a determination of the amount of the deflection is rendered impossible.
Faults in the glass usually have a core (bubble, inclusion). Since the core of a glass fault will primarily absorb light, a measuring of the core is possible only within the bright field 5. In the dark field 4, no measurement of the core is possible.